Surging due to backflow of fluid when flow volume is small restricts an operating range of a centrifugal compressor for raising pressure of compressible fluid. Since surging makes a centrifugal compressor inoperable, an operating range of a centrifugal compressor can be expanded by restricting surging. A Patent Document 1 listed below discloses casing treatment as one of methods for restricting surging.
A centrifugal compressor includes an impeller that rotates at high speed, and a casing that houses the impeller while forming a scroll flow passage around the impeller. In the casing treatment disclosed in the Patent Document 1, a slot is formed along an entire circumference of a wall surface of the casing near an upstream end of the impeller, and the slot is communicated with a flow passage on an upstream side from the impeller. According to this, fluid partially recirculates from a high-pressure portion locally generated in the impeller when flow volume is small to the upstream side from the impeller, so that surging is restricted.